This invention relates to the low-cost manufacture of a tunable physical topographic pattern and more particularly to the manufacture of micro and nano scale hierarchical periodic wrinkle patterns that are generated upon compression of supported thin films.
Micro and nano enabled devices have the potential to significantly impact diverse fields with direct societal benefits such as energy, water, health, and environment among others. These devices function by actively manipulating matter and/or energy on the micro/nano length scale and often rely on the structure-property relationship to achieve this manipulation. This active manipulation enables using micro/nano enabled products in applications such as (i) fluidics based medical diagnostics, (ii) high-sensitivity sensing of toxic chemicals, and (iii) optoelectronics based chemical and biological sensing. As these devices rely on the structure-property relationship, several different properties can be simultaneously controlled by incorporating different types of structures on the same device. One of the techniques to achieve this is via fabrication of hierarchical structures. A hierarchical structure is one that comprises features on multiple length scales and demonstrates “nested features”, i.e., a set of features built on top of another set of features. Each of these set of features may be used to control a different material property. For example, large-scale features in a hierarchical structure may be used to direct/manipulate fluid flow whereas small-scale features may be used to tune the local adhesion/stickiness. Thus, low-cost fabrication of hierarchical micro/nano structures is essential if one desires affordable manufacturing of multi-function micro/nano enabled devices.
Current processes for fabricating micro/nano scale hierarchical structures are primarily limited in terms of the (i) cost and scalability of fabrication and (ii) tunability of hierarchy. At present, hierarchical micro/nano structures are fabricated via a combination of two or more substantially different fabrication processes. This leads to manufacturing challenges in terms of throughput, cost, and/or scalability as one needs to satisfy the requirements for multiple processes. Additionally, it is infeasible to tune/modify the hierarchy after the patterns have been fabricated. For example, it is currently not possible to deterministically switch a pattern across non-hierarchical and hierarchical states or to change the relative “strength” of the individual patterns within the overall composite pattern. This inability to tune the hierarchy prevents one from applying hierarchical structures to build tunable “smart” sensors and devices. Thus, there is a need to develop fabrication processes for scalable and affordable manufacturing of tunable hierarchical micro/nano scale structures. Herein, a scalable and affordable process to fabricate tunable hierarchical structures via a single fabrication process is disclosed. This fabrication process was developed by performing wrinkling of pre-patterned surfaces wherein the pre-patterned surfaces are also fabricated via wrinkling.
Wrinkling of thin films is an affordable and scalable process for fabricating periodic sinusoidal patterns over large areas. Wrinkled patterns are formed on supported thin films as a result of buckling-based instabilities and the mechanism is similar to Euler buckling of beams under compressive loads. A schematic of this process is illustrated in FIG. 1. Essential elements of a system that demonstrates wrinkle formation are: (i) a film 10 that is thin relative to the base, (ii) mismatch in the elastic moduli of the film and the base 12 with the film being stiffer than the base, and (iii) loading conditions that generate in-plane compressive strain (ε) in the film. In such bilayer systems, the state of pure compression becomes unstable beyond a critical strain and wrinkles are formed via periodic bending of the film/base. The period of wrinkles (λ) is determined by the competing dependence of strain energy on period in the film versus in the base. The amplitude (A) is determined by the amount of applied compressive strain. Several different techniques have been developed in the past to (i) generate and join/bond the film to the base, (ii) generate moduli mismatch, and (iii) apply uniaxial and biaxial strains to the film. During compression of flat/smooth films, one is limited to a single period wrinkled pattern even with all of these different combinations of techniques. Thus, to obtain hierarchical wrinkled patterns one must start with non-flat film geometry.
Although fabrication of hierarchical wrinkled patterns has been demonstrated in the past, current techniques for wrinkling have major limitations that prevent one from using these techniques in a manufacturing environment. These limitations are: (i) inability to accurately predict the resulting pattern for a given set of process parameters and (ii) inability to perform inverse pattern design; i.e., inability to predictively design and fabricate the desired hierarchical patterns by combining several patterns. Thus, using current techniques one can fabricate some form of hierarchical wrinkles but not the desired targeted hierarchical pattern. This makes it impossible to use the current techniques to (i) deterministically switch between hierarchical and non-hierarchical states and (ii) predictively tune the relative strength of the individual periodicities in the composite pattern.
Herein, a technique to deterministically tune the hierarchy of a wrinkled surface is disclosed. The technique is based on the discovery that the hierarchical form during compression of a non-flat bilayer emerges with increase in the compressive strain. This emergence phenomenon has been exploited here to design and fabricate wrinkled surfaces with tunable hierarchy wherein the hierarchical form is tuned via the applied compressive strain. A schematic representation of emergence of hierarchy with compression is illustrated in FIG. 2. This disclosure presents: (i) the process scheme for fabricating hierarchical patterns 22 from wrinkling of pre-patterned surfaces 16, (ii) the tools that enable controlling the parameters during the fabrication process, and (iii) model-driven design of such bilayer systems that demonstrate tunable hierarchy. In combination, these tools and techniques enable one to (i) predictively design and fabricate hierarchical patterns at 1/10th of the cost of the existing processes and (ii) deterministically tune the hierarchical form.